1. Field of the Invention
The present invention relates to a planetary gear apparatus capable of transferring rotation from an input side to an output side while changing gear ratio in stepless manner from the deceleration range to the acceleration range, and especially to a planetary gear apparatus that is used as a variable gear ratio transmission mechanism for changing the correspondence between operation quantity of a steering member and operation quantity of a steering mechanism, for example in a steering apparatus of a vehicle.
2. Description of Related Art
A vehicle is steered by transferring the operation of a steering member such as a steering wheel to a steering mechanism via a steering shaft and by changing the direction of steerring control wheels (in general, right and left wheels) by the operation of the steering mechanism. In such steering of the vehicle, a good steering feel can be obtained by changing the correspondence between the operation quantity of the steering member and the operation quantity of the steering mechanism according to the travel state of the vehicle such as the level of vehicle speed, the amount of steering angle.
For example, by making larger the operation quantity of the steering mechanism than that of the steering member at a low speed travel or middle speed travel, the driving operation can be easier as the steered angle of the wheel gets larger. In contrast, at a high speed travel, by making smaller the operation quantity of the steering mechanism than that of the steering member, the stability is heightened as the steered angle of the steerring control wheel gets smaller since the operation quantity of the steering mechanism is smaller even in a quick operation.
Conventionally, because of above-mentioned circumstances, a steering apparatus was developed such that the rotation transfer from the operation member side (input side) to the steering mechanism side (output side) is performed by changing the gear ratio by interposing a variable gear ratio transmission mechanism in the middle of a steering shaft connecting the steering member and the steering mechanism. It is preferable that a variable gear ratio transmission mechanism could change the gear ratio in stepless manner from the deceleration range to the acceleration range and be constituted compactly. As means for responding to these requests, a variable gear ratio transmission mechanism using a planetary gear apparatus has been put to practice use (for example, see ‘Nikkei Mechanical’, No. 585, pp. 24-25, Jun. 1st, 2003).
The conventional planetary gear apparatus as described above comprises an input sun gear and an output sun gear, which are supported coaxially, and a plurality of input side planetary gears and a plurality of output side planetary gears, which are respectively meshed with the outer peripheries of the input and output sun gears and supported on a common carrier. The apparatus further comprises a speed change motor applying rotation power to the carrier in both forward and reverse directions.
By constituting the conventional planetary gear apparatus in such a manner, the rotation of the input sun gear is transferred to the output sun gear together with the rotation of the input side planetary gears and the output side planetary gears when the carrier is fixed by maintaining the speed change motor in a non-rotation state. Accordingly, the output sun gear rotates at the same speed as the input sun gear, causing the same speed transmission state with the gear ratio of 1.
In contrast, when the carrier is rotated in the same direction as that of the input sun gear by rotating and driving the speed change motor, the rotation of the input sun gear is transferred to the output sun gear together with the rotation of the input side planetary gears and the output side planetary gears, which revolve with the carrier. Accordingly, the output sun gear rotates at the rotation speed of the carrier, that is, the rotation speed added with the revolution speed of the input side planetary gears and the output side planetary gears, causing an increased speed transmission state with the gear ratio larger than 1. On the other hand, when the carrier is counter-rotated by rotating and driving the speed change motor, the output sun gear rotates at the rotation speed decreased by the rotation speed of the carrier from that of the input sun gear, causing a decreased speed transmission state with the gear ratio smaller than 1.
By interposing the conventional planetary gear apparatus as described above in the middle of a steering shaft, connecting the input sun gear to a steering member, and connecting the output sun gear to a steering mechanism, the planetary gear apparatus can be used as a variable gear ratio transmission mechanism as described above and correspond to the change control of steering property according to the travel state of a vehicle. In the planetary gear apparatus as described above, when the speed change motor is driven and rotated without rotation of the input sun gear, the steering mechanism can be operated without the operation of the steering member since the output sun gear meshing with the output side planetary gears revolving with the carrier rotates. This means that corrective steering for correcting an attitude change of a vehicle caused by disturbance such as cross wind, slipping, etc. be also possible.
The conventional planetary gear apparatus as described above is a useful, excellent device as a variable gear ratio transmission mechanism in a steering apparatus of a vehicle since the gear ratio can be varied in stepless manner from the deceleration range to the acceleration range. In an actual use of such a variable gear ratio transmission mechanism, however, the rotation direction is frequently switched for conversion of steering in both right and left directions. As a result, when switching the rotation direction, this apparatus has a problem that unusual sounds might be generated at a meshing portion between the input sun gear and the input side planetary gears and that between the output sun gear and the output side planetary gears.
Such unusual sounds are collision sounds caused by that the teeth of each gear relatively move within a backlash range and collide with each other when switching the rotation direction, and can be eliminated by constituting so as not to generate backlash at each meshing portion. However, the apparatus has a problem that the number of machining and assembling man-hours might be increased since a high level of machining accuracy is required to the input sun gear, the output sun gear, the tooth surfaces of the input side planetary gears and the output side planetary gears, as well as support portions of theses gears for assembling the apparatus in such a manner.